* read() is now readinto() and takes the buffer to write into.
* readinto() returns the number of valid samples.
* readinto() can be interrupted by ctrl-c.
* readinto() API doesn't support signed numbers because it never did.
* sample_rate is now required in the constructor because supported
values will vary per-port.
* 16 bit values are full range. 12 bit samples from RP2040 are stretched
in the same way they are for AnalogIn.
Fixes#7226
